I can't sign in to the DSM WebServices

You may be attempting to sign in to the DSM WebServices with your Google Account sign-in information. You need to use your legacy DART Sales Manager (DSM) sign-in to access the WebServices  even after you update your DSM sign-in to a Google Account. In other words, you'll use your Google Account to sign in to DSM, and your legacy DSM sign-in to access the WebServices.